For a given use, for example for medical applications, nitrogen in liquid/vapour equilibrium is stored in a tank with a predetermined nominal level of liquid nitrogen depending on the use.
Such a tank is generally equipped with a probe of the aforementioned type, placed vertically in order to measure the liquid level and optionally to regulate this level to the predetermined nominal value. A probe of this type is conventionally incorporated into a capacitive measurement device comprising an electronic processing unit which incorporates the voltage supply. This electronic unit is designed to determine the height of the liquid in the tank from the value of the capacitance of the measurement capacitor of the probe since, because of the different dielectric constants of the liquid and gas that are contained in the tank, the capacitance of the capacitor varies proportionally to the height of liquid between its plates and therefore depends on the level of liquid in the tank.
Generally speaking, for a given tank, the probe must be able to measure a whole range of nominal liquid levels. This range comprises a low nominal level of liquid, for example in the case in which the user wishes to store biological products in gaseous nitrogen in the tank, and a high nominal level of liquid, for example in the case in which the user wishes to store biological products in liquid nitrogen in the tank.
Consequently, the height of the plates of such a probe is of the same order of magnitude as the maximum storage height in the tank in question. Thus, different probes are necessary for tanks having different maximum storage heights.